1. Field of the Invention
This invention relates generally to a tone generator for an electrical music instrument or synthesizer, and more particularly is directed to an improved tone generator for an electrical music instrument or synthesizer of the single tone-type, that is, one in which, at any time, only a single tone signal or frequency is generated in correspondence to a key-operated switch which is then actuated.
2. Description of the Prior Art
Electrical music instruments or synthesizers have been provided which include a plurality of keys arrayed to form a keyboard, and a tone generator which generates tone signals corresponding to the keys which are selectively actuated or operated. In existing electrical music instruments, the tone generator includes a voltage divider connected with a DC power supply and with a plurality of switches actuable by respective keys of the keyboard to provide an output voltage corresponding to the position of the operated key in the keyboard. The voltage thus obtained is sampled and held to provide a corresponding substantially stabilized DC voltage which is supplied to an anti-logarithm function or exponential signal generator. The anti-logarithm function or exponential signal generator is designed to convert the DC voltage signal which varies linearly in dependence on the position of the respective operated key in the keyboard to a DC voltage signal which varies in accordance with the frequencies of the twelve tone steps comprising one octave. The resulting voltage from the anti-logarithm function or exponential signal generator is applied to a voltage controlled oscillator so that the latter provides an output oscillation or tone signal having a frequency determined by the switch which is selectively actuated by operation of the respective key. The output oscillation is then amplitude modulated by a suitable envelope signal which determines the quality of the synthesized tone.
The conventional tone generator for an electrical music instrument, as described above, has a number of disadvantages. More specifically, the described tone generator is susceptible to misoperation by reason of possible chattering of the switch which is actuated for selecting the output frequency or tone. Moreover, the anit-logarithm function or exponential signal generator used in the conventional tone generator employs the exponential function characteristic or relation to the base-emitter voltage to the collector current (V.sub.BE -I.sub.C) of a transistor, which characteristic varies with changes in temperature. Thus, the output frequency or tone obtained in response to the operation of a selected key of the keyboard may vary with changes in ambient temperature.
Further, when playing an electrical music instrument of the single-tone type, there is likely to be some overlapping of the periods during which successively operated keys are depressed, on other words, at any one time two or more keys may be depressed so as to simultaneously actuate the respective switches. In such case, the conventional tone generator for a single tone electrical music instrument will always give priority to either the higher or lower one of the output tones or frequencies respectively corresponding to the simultaneously actuated switches. In other words, if the conventional tone generator is designed to give priority to the lower tone or frequency and the operator first operates a key corresponding to a lower tone and then operates or depresses a key corresponding to a higher tone without fully releasing the earlier operated key, the relatively lower tone or frequency will be reproduced during the simultaneous operation of both keys. Contrary to the foregoing, in a single tone electrical music instrument, it is desirable that the output from the music instrument always correspond to the latest operated key. The conventional tone generator for an electrical music instrument is still further disadvantageous in that the envelope signal by which the output tone or frequency is amplitude modulated for determining the quality of the output oscillation or tone signal may not be reliably produced during legato playing of the instrument.